Depositional Framework of the Upper Cretacous Mancos Shale And Lower Part of the

Upper Cretaceous Mesaverde Group, Western Colorado and Eastern Utah

By

Ronald C. Johnson

USGS, Denver, CO

A detailed history of subsidence and infilling patterns was reconstructed for the Mancos Shale and lower part of the Mesaverde Group in western Colorado and eastern Utah. The interval was deposited in offshore and marginal marine settings. Rates of subsidence for the lower part of the Mancos increased toward the northwest toward a deep trough southeast of an area of active thrusting within the Sevier orogenic belt. A relatively stationary, southeast sloping sub-sea ramp developed in this trough, with the locus of deposition shifting through time. Beginning in Campanian, (Mancos B interval), a marine shelf prograded from south to north across the southern part of the study area. The shelf eventually encountered the ramp structure to the north, and progradation stopped. Sandstones were deposited in the topographic low between the ramp and the shelf. New subsidence patterns developed in late Campanian with subsidence rates increasing toward the east and decreasing toward the north part of the study area. These changes initiated a period of infilling on the ramp. Southeast prograding regressive marine cycles were deposited after the infilling. These new trends are similar to Laramide trends but predate the onset of uplift on the Laramide White River uplift to the east.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming